As used here, a fluidic channel refers to a channel configured to carry a fluid in a substrate. Some devices require that fluidic channels be connected by a functional material different from a substrate material used to form the channels. For example, in some Joule-Thomson (JT) cryocoolers, in which a gas under pressures adiabatically expands through a nozzle into a chamber of lower pressure, the high pressure gas is thermally conditioned before entering the chamber by the temperature of the low pressure exhaust gases, a design known as heat exchange. A cryocooler implies a device designed to cool to very low temperatures, such as −150 degrees Celsius (° C.) or 130 Kelvin (K), and below. The thermal conditioning is accomplished, for example, through a thermally conductive material (as the functional material) to support heat exchange. The thermally conductive material is different from the material serving as a substrate for channels, which is thermally insulating in order to support the adiabatic expansion.
When the fluidic channels are on the microscale (cross sectional dimensions from about 1 to about 1000 microns, 1 micron=10−6 meters) or nanoscale (cross sectional dimensions from about 1 to about 1000 nanometers, nm, 1 nm=10−9 meters) fabrication become challenging. In such cases, the functional material is often formed into a second layer, separate from a wafer serving as the substrate for the fluidic channels. A cover for the channels, with any reservoirs or access ports, is then formed in a third layer. The multilayer fabrication introduces complexity and expense in having three or more fabrication configurations and introduces challenges in alignment of the separately fabricated layers.
For example, some microscale JT cryocoolers have been fabricated using Micro-Electro-Mechanical Systems (MEMS) or Nano-Electro-Mechanical Systems (NEMS) micromachining, and semiconductor processing methods. These fabrication methods involve the use of three or more wafers as substrates to achieve the effective integration of fluidic circuits, and do not allow for the integration of thermally conductive material useful for such thermal conditioning as in a regenerative cooling design. The fabrication techniques involved (e.g., deep reactive ion etching, microparticle sand blasting, selective laser ablation) are highly complex, specialized, expensive, and often difficult to maintain in a manufacturing mode.